Physics
Cambridge IGCSE

TOPIC 3: WAVES

# 3.5 The electromagnetic spectrum

In this section we will learn about how the colours that we see in light from the Sun are not the only waves radiating all around us. There is a huge spectrum of other waves - similar to light - that our eyes cannot detect:

## Beyond the visible spectrum

As we met in section 3.3, a prism can refract white light and disperse the beam into the colours of the spectrum. The spectrum is traditionally described as consisting of 7 different colours. The red end of the spectrum is the light wave with the longest wavelength and lowest frequency. Violet light is the part of the visible spectrum with the shortest wavelength and the highest frequency. However, scientists have found that some animals can see light with a shorter or longer wavelength than visible light:

For example, bees can see light with a much higher frequency than violet light. This light is called ultraviolet light. Snakes, on the other hand, can detect light with a longer wavelength than red light, called infrared light.

Light can pass through a vacuum - completely empty space. The light wave is actually an electric and magnetic vibration in space. As visible light, ultraviolet and infrared light all have identical properties, they are all called electromagnetic waves. They form parts of the electromagnetic spectrum. This family of waves extends far beyond that which any animal eyes can detect, as figure 1 shows:

Figure 1. The electromagnetic spectrum

You may well recognise some of the types of waves in the figure above. Even though the behaviour of these waves are quite different, they are actually all in the same family.
Note that is some books this diagram is reversed.

### Properties of E.M. waves

All of these waves have some key identical properties:

• They all travel at the same speed (the speed of light) in a vacuum. This is 3 x 108 m/s, and the speed in air is about the same value.
• They are all transverse waves.
• They are all electromagnetic waves.
• They can all be reflected and refracted.

Learn these key properties that show they are all in the same family. You will also need to know a bit more about each of the types of waves shown in figure 1, as described here:

Radio waves are low energy waves that travel through many objects quite easily. They are used for communications because of these properties. They can pass though thin concrete, glass, and our own bodies. FM and AM radio stations that we often listen to in cars and at home are 'broadcasted' using standard radio waves to send the signals to us.
To produce a radio wave we need a transmitter - these produce radio waves from an alternating current passed through them. We need an aerial to receive the radio waves, converting them to an electrical signal. and then a circuit to decode the signals. Two-way radios communicate with radio signals, as the name suggests. Television signals around the world are transmitted using radio waves, although this technology is rapidly being replaced by high speed internet transmission.

Radio frequency identification tags ('RFID' tags) are small devices that can be fitted to cars, parcels and a range of other items. When a radio wave is emitted by a nearby transmitter, the tags respond with an identification code. This means the tag can be tracked, really useful for cars going through tolls and for parcels being tracked as they are transported. In figure 2, the transmitter is above the cars as they drive past the RFID check point.

In addition, radio waves can also be used in astronomy, as many objects in space emit radio waves. This can be useful as radio waves can penetrate through dust clouds more effectively than visible light. Figure 3 gives a radio image of the centre of our own Milky Way galaxy, showing details not seen with visible light telescopes.

Figure 2: An RFID toll lane.

Figure 3: A radio telescope image of the Milky Way centre
To allow for easy communication between devices at home, a radio signal system called Bluetooth has been developed. "luetooth uses radio waves because radio waves pass through walls but the signal is weakened on doing so. This is why you often need to be in the same room for Bluetooth connected devices to work properly.

Radio waves are all around us and we do not notice them at all. They are not dangerous at typical strengths because of their low energy.

## Microwaves

These are similar to radio waves, but - as the name suggests - they have much smaller wavelengths. They are also used for communication, and are used in all mobile phones. (Cell phones). They can carry more information in the signal than radio waves, but need a more direct line of sight to link with your phone. Microwaves are used to communicate with satellites orbiting the Earth, - allowing us to receive T.V. signals, and also G.P.S. signals to show our location on our phones.

At home, wireless internet also uses microwaves for a similar reason, allowing high speed data transmission. Mobile phones and wireless internet also use microwaves because microwaves can penetrate some walls, and only require a short aerial for transmission and reception. Infrared radiation has a higher frequency, but could not pass through walls to allow for easy communication.

There is one specific frequency of microwaves that makes water molecules vibrate. This means that it will heat up water in food, and this is the technology used in microwave ovens. Note that it is only a specific frequency that is utilised - there are many fake videos available that show mobile phones used to cook food! This cannot happen, as mobile phones use a different frequency from microwave ovens. However, the right frequency of microwaves could cause internal heating of body tissues and for this reason we should avoid exposure to strong microwave transmitters.

## Infrared light

If you stand in front of a large fire, you can feel the heat radiation on your skin coming from the fire. All heat radiation is in fact infrared radiation. This heating effect is used in appliances like electric grills, toasters and electric fires.

Some camera lenses can detect infrared, and this can be used to great effect in security cameras. An area can be lit with a strong infrared light source, but still still look dark to us as our eyes cannot see these long wavelength waves. As our bodies are at 370C, we radiate a small amount of infrared heat all the time. This can be detected with very sensitive cameras -another 'night vision' thermal imaging technology that uses infrared light. The heat from our bodies can be detected by infrared intruder alarm systems, looking for unwanted visitors in secure places like banks and jewellry shops.

Infrared light can be used in optical fibre systems as well as light. See section 3.2b for more about optical fibres.

All of our remote controls use infrared light to send signals to our electrical devices like televisions. In the YouTube clip, you can see several features of infrared light being displayed. These include the fact that the camera can 'see' the infra red signal, and display it on our screens as visible light. We can also see the infrared light being reflected.

YouTube - infrared light in remote controls

The only danger of infrared is the heating effect - high levels of infrared could potentially burn our skin.

## Visible light

We should all be familiar with some basic applications of visible light. We can use light rays for photography, and obviously for our own vision. Creating visible light for our own use at night is a global industry, and the development of highly efficient LED lighting has led to considerable recent developments in the illumination industry.

One of the most useful recent developments is the use of light rays for communications using optical fibres. The light ray can be switched on and off extremely quickly, and therefore used to send a high rates of data in a very short time. This application is speeding up communication through the internet and allowing all of our digital devices at home to receive information down one fibre. Optical fibres are made of high quality glass, and can be used for cable television and high-speed broadband because glass is transparent to visible light and some infrared. See section 3.2b for more about optical fibres.

In terms of harmful effects, very strong visible light can potentially cause skin burns.

## Ultraviolet light

Ultraviolet (sometimes shortened to U.V. light) is shorter wavelength than visible light and has more energy. This can be dangerous as it can damage our eyes and cause blindness, and also cause burns to our surface skin cells. The Sun produces U.V. light, and it is this that can give us sun burn if we stay out in the Sun for too long.

However, U.V. light also has some useful applications. Some chemicals absorb U.V. light, and then re-radiate it as visible light. This means that they chemicals 'glow' under a U.V. source, an effect called fluorescence. (Figure 2). Fluorescent chemicals are used in bank notes making them harder to forge. To check the note is genuine, the note is held under a U.V. lamp to see the hidden images or text from the fluorescent inks. This can be applied to a wide range of security markings including expensive sports or concert tickets.

Figure 4. Fluorescent chemicals

If water flows past a strong source of U.V. light, microbes are killed, and therefore U.V. light can be used to sterilise water for safe drinking.

## X-rays

These rays are much higher energy waves than U.V. light. They can therefore penetrate through our skin. This, though, is one of the key applications of x-rays in that they can pass through us and out of the other side, only being stopped by dense bones. By taking a picture of the x-rays that emerge, we can 'see' our skeletons and other features inside the body.

This technology is also used for security in airports, to take a picture of what is inside our luggage in an X-ray scanner. We can also detect internal breaks or defects inside materials using this scanning technology, for example to check that an aircraft wing is still strong and has no cracks or other flaws.

X-rays can be dangerous and we must minimise exposure to them. Doctors, nurses and technicians in hospitals must not be exposed to too much radiation over a lifetime. X-rays can cause internal cell damage, organ damage and can mutate cells, causing cancer.

## Gamma rays

These are the waves with the highest frequency in the electromagnetic spectrum, and are therefore the most dangerous. Gamma rays can damage cells, killing them, or producing mutations which lead to cancer.

However it is this effect that can be utilised for our benefit. Food, and medical equipment that has been exposed to strong gamma rays will be sterilised -all microbes on the surface will be killed. Food will stay fresh for much longer - the gamma rays do not affect the taste or the chemical composition of the food. Medical equipment is made much safer and this prevents the spread of disease in hospitals.

Gamma rays can also be used in medical tracers that can detect cancers in the body. By directing gamma rays at a cancer growth, the cancer cells can be killed, whilst more resilient healthy cells around the tumor can recover. This treatment is called radiotherapy.

## Summary

You will need to be able to explain some uses and some dangers for each of the sections of the electromagnetic spectrum. Table 1 below gives a quick checklist to help you remember the key points required for this course:

Region of E.M. spectrum Uses

Dangers

microwaves mobile (cell) phones, satellite communication, microwave ovens internal heating of body cells
infrared electric grills,thermal imaging, optical fibres, intruder alarms skin burns
visible photography, vision,illumination, optical fibres skin burns
ultraviolet fluorescent lamps and inks for security marking and bank notes, sterilising water damage to surface cells and eyes, leading to skin cancer and eye conditions
x-rays observing internal structures,
for medicine and materials
mutations of cells, damage to internal cells and organs, cancer
gamma rays sterilising food and medical equipment, detection of cancer as well as treatment (radiotherapy) mutation of cells, damage to internal cells and organs, cancer

Table 1. Uses and dangers of the electromagnetic spectrum

Questions:

1. Fluorescence can be used to protect bank notes from being forged.

• a) Which region of the electromagnetic spectrum causes fluorescence?
• b) Describe how fluorescence is used to identify genuine bank notes.

a) Fluorescence uses ultraviolet light.

b) U.V. light from a lamp is directed on the notes. Fluorescent inks /chemicals in the note absorb the U.V. and re-radiate it as visible light. This causes the inks / chemical to glow, known as fluorescence, showing images or text that was previously hidden.

2. A microwave transmitter produces a wave that has a wavelength of 5 cm.

• a) Describe one application of microwaves.
• b) Describe one possible detrimental effect of exposure to microwaves.
• c) Calculate the frequency of the microwave.

a) Microwaves can be used for heating food / satellite communications / mobile phone communications.

b) Microwaves can potentially cause a heating effect inside the body.

c) Converting the wavelength of 5 cm to metres gives 0.05 m.
The speed of all electromagnetic waves should be learned: It is 3 x 108 m/s.

We know that v = ƒ x λ, so the frequency ƒ is:

 ƒ = v λ
 ƒ = 3 x 108 0.05

ƒ = 6 x 109 Hz (6 GHz)

## Communications

Much of our development in technology has been accelerated by the way we can communicate over large distances, sharing ideas and findings with the scientific community. A vast entertainment industry relies on fast, high data rate communications. In this section, we will learn how information can be transmitted in a range of methods.

### Satellite communications

Artificial satellites have been placed in orbit around the Earth, allowing us to communicate with each other and receive communications and data from the satellite. There are two main ways the satellite can be used to do this:

### 1. Geostationary orbit satellites

If you put an artificial communications satellite in orbit about 36 000 km above the Earth's surface, it will  have a time period of 24 hours, orbiting once per day. However, if placed above the equator, this means it will stay above the same point on the Earth for as long as it remains in this orbit. This is really useful for satellite phones and satellite TV stations, as the user will always have the signal from the same point in the sky, without variation. These satellites use microwaves, as they have a higher frequency than radio waves and can therefore send information at a higher rate.

Figure 5: A satellite in geostationary orbit

### 2. Low orbit satellites

These satellites can be much closer to Earth and therefore to the user, and this is much more convenient for satellite phone use. However the satellite has a much smaller circumference orbit, and takes only a few hours to travel around the Earth. This means that multiple satellites must be launched so that at least one is always visible to the phone user.

### Analogue vs. Digital Signals

Music, photos and videos can all be transmitted over the internet or using Bluetooth radio signals. But how is this done? How can information be sent at such a high speed? To understand this, we need to know how digital signals are created and sent.

All music is made up of sound waves. If we send the original sound wave from one device to another, perhaps using radio waves instead of sound waves to show the shape of the wave, then we are sending an analogue signal. This is a signal that can take any value it wants. In maths terminology, the displacement of the wave at any point is known as a continuous variable. However this wave can be measured, and the height of the wave at each point in time can be transmitted as a number. To do this the height ('displacement') is measured, but this means some rounding up or down will occur, and the wave becomes a digital signal, which is a discrete measurement, only taking a limited number of values. Figure 6 shows how this affects the shape of the wave and compares analogue and digital waves.

Figure 6: Analogue and digital waves compared

Looking at figure 6, on first appearances turning the wave into a digital signal appears to add some 'distortions' to the wave. This is one reason that some music purists prefer listening to original vinyl records where the recording is an analogue signal.

However there are two reasons that digital communications has taken over from analogue in nearly all forms of communication:

### 1. Data speeds

Once the signal has been converted into a measurement, this can be converted into binary code by a computer, and sent via microwave signals, electrical cables or optical fibres at incredibly fast transmission rates. A digital file of a feature film can be transmitted in seconds using some of the fastest systems commercially available. This means our home internet can cope with multiple users, all transmitting and receiving packets of digital code at high speeds. Our devices such as phones and computers then convert these packets back into - for example - the analogue music wave that we want to listen to.

### 2. Signal Distortion

When a radio signal is transmitted, the receiver can pick up additional 'noise' from other sources around us. This can distort the signal and lead to music sounding 'crackly' or videos having poor quality pictures. However digital signals do not have this problem. In figure 7, noise has been added to an analogue and a digital 'square wave' signal that represents the 1's and 0's used by binary code.

Figure 7: Analogue and digital waves with added noise

The analogue signal is now distorted, and it is not possible to work out the original wave pattern from the received signal. However, for the digital signal, a computer processor could easily identify the 1's and 0's from the distorted signal shown in the bottom right diagram of figure 7, and regenerate the original 'clean' signal. This accurate regeneration of the original code allows digital signals to be used for many applications with excellent quality sound, pictures and video.

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